Field of the Invention
This invention relates to thin film polymeric dye compositions and more particularly to such compositions prepared for nonlinear optical properties by means of the Langmuir-Blodgett process.
The field of nonlinear optics is concerned with the interactions of electromagnetic fields in various media, such as the thin polymeric films, to produce new fields altered in phase, frequency, amplitude, or other propagation characteristics from the incident field. See, for example: Y. R. Shen, "The principles of Nonlinear Optics", John Wiley & Sons, New York, 1984. The best known nonlinear effect is second harmonic generation (SHG) or laser frequency doubling. Optical nonlinearity is useful for doubling the frequency of lasers, for electro-optical interconnect devices, other optical devices and laser resistant devices.
Here-to-fore, nonlinear optical devices, such as laser frequency doublers, have been based exclusively on crystalline inorganic materials, such as lithium niobate and potassium dihydrogen phosphate. Disadvantages of these inorganic materials include slow response times to optical signals, poor laser damage resistance, and small optical nonlinearities. Additionally, these inorganic materials are difficult to prepare and process into microelectronic devices. Nonpolymeric organic materials, on the other hand are limited by poor mechanical properties and the difficulty of growing large, high quality single crystals. Organic nonlinear optical materials are described in: A. F. Garito and K. D. Singer, Laser Focus, 18, 59 (1982); D. J. Williams, ed., Am. Chem. Soc. Symposium Series 233 (1983); and D. J. Williams, Angew. Chem. Intl., 23, 690 (1984).
It has been found that organic polymeric materials with large delocalized .pi. electron systems (called dyes or chromophores) exhibit large molecular hyperpolarizabilities, which translate into large nonlinear optical responses when the dipole moments of dye groups are aligned, on the average, in the same direction. Polymeric materials have good mechanical properties, are easily processed, and can have thin-film geometries desirable for coatings and integration with microelectronics. It is important to note that piezoelectric and pyroelectric properties are also potentially exhibited by these films. These properties are useful in pressure and temperature sensors, respectively.
There are many techniques for depositing thin polymer films including adsorption from solutions (see L. R. Netzger and J. Sagiv, Thin Solid Films, 132, 153 (1985), and B. Vincent and S. G. Whittington, Surf. and Colloid Sci., 12, 12 (1982); plasma deposition (see H. Yasuda, J. Polym. Sci.; Macromolecules Rev., 16, 199 (1981); electrodeposition (see B. K. Garg, et al., AIChE J., 22, 65 (1978); and simply casting from solution. Although some surface order may exist in polymer films made by these techniques, none have yet been able to induce persistent order in three dimensions. Electric-field poling of these films while in a liquid state then cooling to the glassy state can form polarized polymer films. (See C. Ye, et al., Mat. Res. Soc. Symp. Proceedings, 109, 263 (1988)). However, there are many hazards and disadvantages in working with the large voltages required, such as dielectric breakdown and subsequent destruction of the film. Furthermore, films processed by electric field poling must not contain any mobile ionic species in order to avoid dielectric breakdown.
Another approach to preparation of thin polymer films is the Langmuir-Blodgett deposition technique ((see I. Langmuir and K. B. Blodgett, Kolloid-Zeitschrift, 73, 257 (1935)) which involves depositing a solution of nonvolatile, amphophilic material in volatile solvent onto the surface of highly purified water, evaporation of the solvent, leaving an ultra thin film (ca. 2 to 3 nm thick) of the amphophilic material on the water subphase, compressing the film with a partially submerged, movable barrier or dam, holding the film at a constant degree of compression or surface tension by means of a computer-controlled film balance, and dipping a solid substrate vertically or horizontally into and out of the film-water interface which coats the solid substrate with the amphophilic film material (see G. L. Gains, Jr., "Insoluble Monolayers at Liquid-Gas Interfaces", Whiley-Intersciences, New York, 1966.). Languir-Blodgett film balances or troughs are commercially available from many suppliers. Research in this area is published every year or two from the International Conferences on Langmuir-Blodgett Films (see Thin Solid Films, Elsevier Sequoia, S. A., Lausanne, Switzerland, e.g., most recently in volume 159, May 1988).
Reports on the fabrication of multi-layered Langmuir-Blodgett films comprised of non-polymeric amphophilic dye molecules are presented in: L. M. Blinov, et al., Sov. Phys. Solid State 24, 1523 (1982); M. F. Daniel and G. W. Smith, Mol. Cryst. & Lig. Cryst. 102, 193 (1984); I. R. Girling, et al., Thin Solid Films, 132, 101 (1985); D. B. Neal, et al., IEEE Intl. Symp. on Applications of Ferroelectrics (1986); Th. Rasing, G. Berkovi, Y. R. Shen, S. G. Grubb and M. W. Kim, Chemical Physics Letters, 130(1,2,), 1 (1986); H. Nakasnishi, et al., Japanese J. of Applied Physics, 26(10), 1622 (1987); M. Era, et al., Japanese J. of Applied Physics, 26(11). L1809 (1987); L. M. Hayden, S. T. Kowel and M. P. Srinivasan, Opt. Comm., 61(5), 351 (1987); J. S. Schildkraut, et al., Optics Letters, 13(2) 134 (1988); D. Lupo, H. Ringsdorf, et al., J. Opt. Soc. Am. B, 5(2), 300 (1988). However, we have found that attaching the dyes to polymers makes the films more robust and more able to retain a thickness-dependent quadratic enhancement of second harmonic generation (SHG) than can monomeric amphiphilic dyes.
A number of laboratories have been working independently on attaching dyes to polymers for nonlinear optical applications. However, the polymeric dyes created so far, are not suitable for Langmuir-Blodgett deposition, but rather are formulations suitable only for electric field poling of spun-cast films. See for example: P. LeBarny, et al., Proceedings of the SPIE, 682, 56 (1987); T. J. Marks, et al., Material Research Society Symposium Proceedings, 109, 263 (1987); A. C. Griffin, et al., ibid., p. 115; R. DeMartino, et al., ibid, p. 65. The criteria for selecting polymers are quite different for Langmuir-Blodgett deposition than for the electric field poling process described in these references. For example, ionic charges should be absent for best results in electric field poling because the ions tend to migrate in large electric fields leading to dielectric breakdown of the organic film. For Langmuir-Blodgett deposition, on the other hand, ions in the polymeric dyes impart necessary hydrophilicity and desirably large optical nonlinearity. However, care must be exercised to have the proper balance of hydrophilicity and hydrophobicity.
Mixtures of dyes and polymers (so called guest-host systems) in Langmuir-Blodgett films which exhibit second-order nonlinearity are known in the prior art. See P. Stroeve, et al., Thin Solid Films, 146, 209 (1987), and S. T. Kowel, et al., Opt. Eng., 26(2) 107 (1987). However, these physical mixtures do not give quadratic enhancement of second harmonic generation, and the dye molecules diffuse over long periods of time causing the nonlinearity to decrease.
Work on polymeric dyes designed for Langmuir-Blodgett fabrication into nonlinear optical films and which demonstrated quadratic enhancement of optical nonlinearity as a function of film thickness was reported by the present inventors in: R. C. Hall et al., SPIE Proceedings, 824, 121 (1988), and R. C. Hall, et. al., Materials Research Society Proceedings, 109, 351 (1988). These papers are incorporated by reference herein.
Related publications are: N. Carr, et al., Makromol. Chem., Rapid Commun. 8, 493 (1987), and R. H. Tredgold, et al., Electronics Letters, 24(6), 308 (1988). The Carr paper describes chemically attaching an azo group to a poly(dimethyl siloxane) and measuring second harmonic generation from a single monolayer. Their composition contained a hydrophobic backbone and a weak dipole moment. They report the futility of making multilayered, noncentrosymmetric films by the Langmuir-Blodgett technique, and hence reported results on only one monolayer of polymer. The Tredgold paper describes interleaving a comb-polymer containing a weak dipole moment with a small molecule containing a large dipole moment. They encountered great difficulties in efforts to form thick Langmuir-Blodgett films of alternating layers of two distinct polymers. They did not observe quadratic enhancement of the second harmonic with thickness of their films.
Thus, the prior art describes arrangements of nonpolymeric dyes into fragile, thin films which exhibit optical nonlinearity which increases only linearly with thickness beyond a few layers of thickness, or which rearrange with time to give little or no optical nonlinearity. Additionally, the prior art discloses mixtures of polymers and dyes arranged into nonstable films whose optical nonlinearity increases less than quadratically with thickness and whose nonlinearity also decreases with time. These polymer-dye compositions cannot be fabricated into a multilayer film with the necessary optical properties and characteristics.
It is an object of the present invention to provide novel comb-shaped polymers comprised of chemically attached dyes specifically designed for Langmuir-Blodgett thin film processing.
It is thus an object of the present invention to provide optically nonlinear polymer films having large optical nonlinearities.
It is another object of the present invention to provide noncentrosymetric films having large second order optical nonlinearity.
It is another object of the present invention to provide optically nonlinear polymer films having fast response time to optical signals and good mechanical properties and resistance to laser damage.
It is still another object of the present invention to provide polarized polymer films having a dye group concentration as great as possible maximizing the nonlinear optical effect, and minimizing optical losses and any time dependent relaxation of the non-centrosymmetric molecular dipole orientation.
It is yet another object of the present invention to provide optically nonlinear polymer films easily prepared and fabricated in thin film geometries for coatings and other formats for use in optical devices.
It is yet another object of the present invention to provide a polarized polymer film from the polymers of this invention fabricated by the Langmuir-Blodgett technique.
It is another object of the present invention to provide a polarized polymer film from polymers of this inventions which produces a quadratic enhancement of second harmonic generation through at least five Langmuir-Blodgett monolayers of polmer-bound dye.
It is still another object of the present invention to provide a multilayered, optically nonlinear, polymer film.
It is another object of the present invention to provide a pyroelectric sensing nonlinear polymer film.
It is finally another object of the present invention to provide a piezoelectric polymer film.